Stabilized semi-submersible platform

ABSTRACT

Angular stabilization and positioning of a semi-submersible platform is achieved through floating supports whose length is substantially greater than their diameter, closed at their base and vertically oriented with a cylindrical horizontal member on which the swell acts fixed to the base to compensate the angular movement it produces on the vertical supports and to position the platform in relation to the direction of its propagation.

United States Patent Monnereau 1 Mar. 21, 1972 [54] STABILIZED SEMI-SUBMERSIBLE Refmm Cited PLATFORM .7 1 UNITED STATES PATENTS [72] Invent: Paris France 2,944,267 7/1960 Kurtz ..9/8 [73] Assignee: Compagnie Generale DAutomatisme, 3,089,157 5/1963 May.... ...l 14/140 X Paris France 3,159,130 l2/l964 VOS ..l 14/05 [22] F] d D 10 1969 3,490,406 1/1970 O'Reilly et al. ..l14/O.5 D

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Prima Examiner-Milton Buchler 1 A 1.14.; 883723- [2 1 pp 0 Assistant Examiner-Gregory W. O'Connor Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak [30] Foreign Application Priority Data CT [57 ABSTRA Dec. 10, 1968 France ..l77486 Angular stabilization and positioning of a semi-submersible [52] U S Cl 9/8 P platform is achieved through floating supports whose length is 51 I .t .Cl substantially greater than their diameter closed at heir base 1 and vertically oriented witha cylindrical horizontal member 58 Field of Search ..9/s R, 8 OP, 114 05, 230, on which the swell acts fixed to the base to compensm the am 114/435, 121, 140, I22, 124, 126; 61/46' gular movement it produces on the vertical supports and to position the platform in relation to the direction of its propagation.

10 Claims, 1 1 Drawing Figures PATENTEDMAR21 1972 3,649,978

SHEET 2 OF 3 I STABILIZED SEMI-SUBMERSIBLE PLATFORM BACKGROUND OF THE INVENTION 1. Field of the Invention disturbing couple until it becomes equal again to the correction couple.

The present invention involves a device for angular stabilization and orientation of a semi-submersible platform sub- The present invention concerns a semi-submersible plat- 5 lected a Swen, including at least one Partially submerged form including semi-immersed supports in the form of closed, hollow vertical cylinders holding the platform out of the water, and relates more particularly to the attachment to each support of means for improving the angular stability of the said platform in a swell, these means having a generally cylindrical form, with their generatrices horizontal. The swell also exerts an orientation action on such a structure.

2. Description of the Prior Art It is known that such structures, particularly in the form of observation posts or platforms for oil-drilling at sea, must be stable in a swell. It is known to employ, for this purpose, the teaching of French Pat. No. 1,510,937, filed by the applicants on Nov. 30, 1966, in disposing around each vertical support a disc element whose dimensions and depth of submersion are chosen so as to cancel the effects of the swell.

The system of forces applied to a semi-submersible platform by the action of the swell reduces to an alternating resultant force whose vertical component gives the platform an alternating translation movement or bobbing motion characterized by its amplitude and frequency. The horizontal component exerts an alternating couple giving the structure an oscillator movement about the vertical, characterized by its angular amplitude and its frequency. Thus, in effect, the swell may be represented as exerting an oriented action on the platform, each point of which oscillates in a substantially vertical plane, parallel to the direction of the swell. This is why the swell does not exert the same effect on all parts of the disc elements mentioned above. In particular, the surface portion of the disc furthest removed from its axis, following the direction of the swell, provide the most important fraction of the correcting couple.

As regards the effects of the vertical bobbing movement, on the contrary, all the elements of the disc surface participate in its production, with no preferential direction.

SUMMARY OF THE INVENTION The invention employs for the stabilization of a platform with semi-immersed vertical supports, a surface extending along a horizontal direction, rather than a disc-like surface having its axis of symmetry vertical. The geometric asymmetry of a surface is thus adapted to the asymmetry of behavior of the two components of the swell, presenting an improved stabilization.

Furthermore, an elongated correcting surface has the property of orienting itself in the direction of the swell, since the asymmetrical action of the swell on the extremities of the said surface generate, in effect, a restoring couple about a vertical axis. The platform' thus adopts a preferred direction with respect to the swell such that when it moves away from this direction, the stabilization surfaces return it thereto. In this stable equilibrium position, the damping of the angular oscillations is improved while the bobbing motion remains the same, being independent of the orientation. In other words, it is possible to obtain with this orientation device the same stabilization of a platform for a bobbing motion less than in case of known devices.

A practical realization of such an elongated stabilization surface consists in using a cylindrical tubular form with horizontal generatrices and preferably open at its extremities. Under the action of the swell, the platform turns until these generatrices are parallel to the direction of the swell, and in these conditions there is obtained, for a given swell, a stabilization of angular movement and heading, with attenuation of the bobbing motion. When the wavelength of the swell changes, the disturbing couple varies and there is no longer angular movement compensation, the position of the platform s center of gravity is varied, by means of counterweights movable along the vertical supports, thus modifying the floating support of elongated form with substantially vertical axis of symmetry, being hollow and closed at its base. Each support is provided with at least one horizontal element of generally tubular form fixed near the base of the support. The device is characterized in that the dimensions and depth of submersion of each horizontal element and the cross-section of each support near the liquid surface are so chosen that the action of the swell on each horizontal element of each support produces a first couple compensating a disturbing couple produced by the swell on the corresponding support, and a second couple tending to orient said horizontal element or elements with respect to the swell; and in that each support is provided with a counterweight adjustable to make the value of the corresponding disturbing couple equal to the corresponding first couple.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1a shows a vertical cylindrical support device provided with a cylindrical compensator element with horizontal generatrices;

FIG. lb is a section on the line and in the direction of the arrows x-x of FIG. 1;

FIG. 2 is a diagram of a compensator cylinder device having an open portion of the side opposite the water surface;

FIG. 3 is a diagram of a vertical support device formed by two cylinders of different diameters;

FIG. 4 is an elevation of a monopod platform with conipensator element formed by three cylinders arranged in starlike configuration;

FIG. 5 is a view from above and in horizontal projection of the compensator cylinders of FIG. 4;

FIGS. 6, 7 and 8 show in horizontal projection the compensator cylinders, in parallel, star-like and triangular configurations respectively, of a device with a tripod support device;

FIG. 9 is a view of a system with two vertical supports (dipode) in which the compensator cylinders are placed in parallel; and

FIG. 10 is a view of a system with two vertical supports, each support carrying a star-like configuration compensator element, these elements being arranged to have a common branch joining the two supports.

DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1a, a hollow cylindrical support 1 is provided at its base with a cylinder 2 with horizontal generatrices and intended to compensate the angular movement produced by the action of the swell on the cylinder 1. The assembly is in equilibrium in water of mean level 3, since the resultant of the buoyancy forces, applied at P, is equal to the resultant of the weights, applied at the center of gravity G. The position of the point G, arranged above the point P so that the equilibrium is stable, depends on the relative positions of the platform fixed above level 3, the cylinder 1, and a counterweight located above the cylinder 2, the platform and counterweight not being shown in the Figure.

In FIG. lb, the cylinder 1b, the cylinder 1 is closed at its lower extremity by the lateral surface of the cylinder 2. The sealing of the cylinder 1 is assured by welding fillets 4, providing the rigid assembly of the two cylinders. The cylinder 2 is preferably open at its two extremities and can be partitioned without prejudicing the orientation of the device about the line x-x by the swell bringing the generatrices of the cylinder 2 into line with its propagation direction.

In FIG. 2, the cylinder 2 is cutaway in that portion opposite the water surface 3, with a window 5 thus eliminating the portion of the surface least subjected to pressure changes caused by the swell. On the other hand, the portions 6 have been kept since at the extremities the lower surface portions of the cylinder 2 are more affected by the swell, in participating in the compensating action.

In FIG. 3, the floating vertical support is in two parts la and lb, the latter being below and having a greater diameter than the former. There is thus obtained a floating support which is more buoyant and able to carry a heavier platform.

However, such supports with a single horizontal cylinder have a tendency, once oriented, to roll about the horizontal cylinder axis. It is then advantageous to provide the horizontal cylinder with a keel 26, FIGS. 1a, lb, attenuating this effect in the known manner. Another way of avoiding this rolling is to divide the horizontal generatrice compensator cylinder into several portions having intersecting axes.

In FIG. 4, the compensator device is divided into three cylinders 7 with horizontal axes spaced in star-like configuration around a vertical support 8, to which they are welded by means of auxiliary corner-plates 9. The platform is shown diagrammatically as closed and provided with openings 11. Counterweights 12 located near the lower extremity of the floating support 8 permit the center of gravity G of the assembly to be brought below the buoyancy center P. The counterweights 12 are fixed to vertical rods 13 movable in guides 14 by a winch 15.

In FIG. 5, the direction of propagation of the swell, that is the direction in which the wave 16 (FIG. 4) moves, is represented by the arrow 17. The orientation action of the swell on the device with star-like configuration of the cylinders 7 involves a rotation of the device through an angle of less than 30 so as to make the axis of the cylinder 7 nearest to the direction 7 coincident with this direction; the other two cylinders 7 arrive in positions'symmetrically disposed about the direction 17.

The dimensions and depths of submersion of the cylinders of the compensation device and the diameter of the floating support having been determined to produce an exact compensation of the disturbing couple, for a particular swell wavelength, any variation of the wavelength produces a modification of the disturbing couple. This disturbing couple, caused by the horizontal component of the resultant of the hydrodynamic forces acting on the floating support, is returned to its basic value by adjustment of the counterweights. In other words, when the force producing the disturbing couple changes, the length of the corresponding lever arm is modified, that is to say, the position of the center of gravity G is modified. The bulk of the compensation system with cylinders arranged in star-like configuration is of substantially the same order of magnitude as that of the equivalent single cylinder.

For the platforms shown above, a single floating support, that is a monopod structure has been used. For platforms of increasing weight, it becomes necessary to provide'several i fl oating supports, giving rise to a multip od structurej The multipod structures are angularly stabilized in the same way as the monopod structures, with horizontal generatrix cylinders; the spatial separation of the compensator cylindersthen gives rise to numerous arrangements.

Thus, three variations are shown for a tripod structure: in FIG. 6 the compensator cylinders 19 mounted on the supports 18 are parallel; in FIG. 7 a star-like configuration of the cylinders 20 has been adopted; while in FIG. 8, the compensator cylinders 27 are triangularly arranged.

There is equally a choice for the compensator cylinder arrangement in the-case of a dipode or biped structure. In FIG. 9, at the base of each floating support 22 is arranged a single compensator cylinder 23, the cylinders 23 being parallel. In FIG. 10, for each floating support 24 a compensator cylinder device has been adopted wherein the compensator cylinders 25 are arranged in star-like configuration, two colinear branches being combined in the form of a common cylinder Calculations and experiments have allowed a determination to be made, for a swell wavelength of 200 meters, of the ranges in which the compensator cylinder device parameters lie.

For a monopod structure with cylindrical support of constant diameter and a compensator cylinder of length L and diameter 1, submerged to a depth h, it being understood that when the compensator cylinder has an open cross-section, the width of the horizontal projection of its surface is represented by I, for a total tonnage between 1 metric ton and 500 metric tons, the ranges are:

d is between 0.4 and 6 meters L is between 5 and 30 meters lis between 2 and 8 meters h is between 4 and 30 meters For example, for a swell direction indicator of one metric ton, d=0.40 m., L=5 m., [=2 m., and H m.; whereas for an observation laboratory of 450 metric tons, the values are the following:

d==4.8 m., L==25 m., i=7 m., and h=27 m.

It should be noted that the invention is not limited to the embodiments and applications particularly described herein, but includes all forms of realization falling within the general definition of the invention which has been given. In particular,

an increase in the tonnage of a monopod platform may be obv tained, for values increasing to 1,000 to 1,500 metric tons, by the following means:

increase of the floating support diameter below the surface (in order to avoid significant increases in the disturbing coucross-section not only a circle but also a curve of any form,

open or closed, or even two separate closed curves corresponding to two cylinders placed in parallel.

The compensator cylinders are shown open at their extremities, but they can be closed without losing their orientation effects. Similarly in the median part of a compensator cylinder of closed cross-section, bulkheads may be installed without influencing the orientation effects due to these extremities.

I claim:

1. In a device for angular stabilization and orientation of a semi-submersible platform subjected to a swell, including at least one partially submerged floating support of elongated form with substantially vertical axis of symmetry, being hollow and closed at its base, and each support being provided with at least one horizontal element of generally tubular form fixed near the base of the support; the improvement wherein: the dimensions and depth of submersion of each horizontal element and the cross-section of each support near the liquid surface are so chosen that the action of the swell on each horizontal element of each support produces a first couple compensating a disturbing couple produced by the swell 0n the corresponding support, and a second couple tending to orient said horizontal element with respect to the swell; each horizontal element is open at its extremities and has a crossseetion in the form of a closed curve, and said device further includes a counterweight for each support, and means for adjusting the position of said counterweight to make the value of the corresponding disturbing couple equal to the corresponding first couple.

2. The angular stabilization and orientation device according to claim I, wherein; each horizontal element has a central portion with a cross-section in the form of a curve open at the side opposite the liquid surface.

3. The angular stabilization and orientation device according to claim 1, wherein: the support is a cylinder and the horizontal element is a single cylinder provided with a stabilizingkeel parallel to its generatrices. Y

4. The angular stabilization and orientation device according to claim 3, wherein: each support is provided with two cylindrical horizontal elements, the sum of the horizontal projections of their surfaces being equal to .that of the single cylinder.

5. The angular stabilization and orientation device according to claim 1, wherein: the support is a cylinder and said cylindrical horizontal elements number three and are equally spaced around the axis of symmetry of said support.

6. The angular stabilization and orientation device according to claim 1, wherein: two supports are provided, each having a cylindrical horizontal element with the elements being parallel.

7. The angular stabilization and orientation device according to claim 1, wherein two supports are provided, each having three cylindrical horizontal elements equally spaced around its axis of symmetry, and one element of each support is part of a cylinder common to and joining the two supports.

8. The angular stabilization and orientation device according to claim 1, wherein said device includes three supports disposed at the apices of a triangle and each is provided with a cylindrical horizontal element with the three horizontal elements being arranged in a triangular configuration.

9. The angular stabilization and orientation device according to claim 1, wherein: three supports are disposed at the apices of a triangle and each is provided with a cylindrical horizontal element extending away from the center of said triangle with the axes of the three horizontal elements being equally spaced around the center of the triangle.

10. The angular stabilization and orientation device according to claim 1, wherein: three supports are disposed at the apices of a triangle and each is provided with a cylindrical horizontal element extending away from the center of said triangle with the three horizontal elements being substantially parallel.

* i i l 

1. In a device for angular stabilization and orientation of a semi-submersible platform subjected to a swell, including at least one partially submerged floating support of elongated form with substantially vertical axis of symmetry, being hollow and closed at its base, and each support being provided with at least one horizontal element of generally tubular form fixed near the base of the support; the improvement wherein: the dimensions and depth of submersion of each horizontal element and the crosssection of each support near the liquid surface are so chosen that the action of the swell on each hOrizontal element of each support produces a first couple compensating a disturbing couple produced by the swell on the corresponding support, and a second couple tending to orient said horizontal element with respect to the swell; each horizontal element is open at its extremities and has a cross-section in the form of a closed curve, and said device further includes a counterweight for each support, and means for adjusting the position of said counterweight to make the value of the corresponding disturbing couple equal to the corresponding first couple.
 2. The angular stabilization and orientation device according to claim 1, wherein: each horizontal element has a central portion with a cross-section in the form of a curve open at the side opposite the liquid surface.
 3. The angular stabilization and orientation device according to claim 1, wherein: the support is a cylinder and the horizontal element is a single cylinder provided with a stabilizing keel parallel to its generatrices.
 4. The angular stabilization and orientation device according to claim 3, wherein: each support is provided with two cylindrical horizontal elements, the sum of the horizontal projections of their surfaces being equal to that of the single cylinder.
 5. The angular stabilization and orientation device according to claim 1, wherein: the support is a cylinder and said cylindrical horizontal elements number three and are equally spaced around the axis of symmetry of said support.
 6. The angular stabilization and orientation device according to claim 1, wherein: two supports are provided, each having a cylindrical horizontal element with the elements being parallel.
 7. The angular stabilization and orientation device according to claim 1, wherein two supports are provided, each having three cylindrical horizontal elements equally spaced around its axis of symmetry, and one element of each support is part of a cylinder common to and joining the two supports.
 8. The angular stabilization and orientation device according to claim 1, wherein said device includes three supports disposed at the apices of a triangle and each is provided with a cylindrical horizontal element with the three horizontal elements being arranged in a triangular configuration.
 9. The angular stabilization and orientation device according to claim 1, wherein: three supports are disposed at the apices of a triangle and each is provided with a cylindrical horizontal element extending away from the center of said triangle with the axes of the three horizontal elements being equally spaced around the center of the triangle.
 10. The angular stabilization and orientation device according to claim 1, wherein: three supports are disposed at the apices of a triangle and each is provided with a cylindrical horizontal element extending away from the center of said triangle with the three horizontal elements being substantially parallel. 